Recommended Crystal, TCXO, and OCXO Reference Manual for High-Performance Jitter Attenuators and Clock Generators (Part 2)

L1 (Motional Inductance) and C1 (Motional Capacitance): L1 and C1 represent the values that comprise the XTAL's electrical LC model. These values determine the resonance frequency and Quality Factor, Q, along with ESR of the XTAL.

CO (Shunt Capacitance): All XTALs have small electrodes that connect the XTAL to the package pins. The electrodes form a shunt capacitance in parallel with the XTAL's LCR model. C0 and C1, along with L1, resonate at a frequency known as anti-resonance fre- quency.

ESR (Equivalent Series Resistance): The equivalent impedance of the XTAL at resonance is the Equivalent Series Resistance. It is mostly dominated by the resistive component R1 given that the ratio of C1/C0 is very small.

For a stable oscillation to take place, the driving oscillator must have a negative impedance 3 to 4 times higher than the ESR of the XTAL. Figure 4.2 shows the maximum ESR allowed to ensure stable oscillation for XTALs in the 48 MHz to 54 MHz range. In this plot, the shunt capacitance C0 is found on the horizontal axis, while the maximum ESR is shown on the vertical axis. To ensure stable oscil- lation, the XTAL must have an ESR below the curve at the maximum C0 specified for that XTAL. Using a XTAL above this curve may not ensure stable oscillation over all conditions.

Figure 3.2. Maximum ESR vs Shunt Capacitance, C0 for 48-54 MHz XTAL

Similarly, Figure 4.3 shows the maximum ESR allowed to ensure stable oscillation for XTALs in the 25 MHz range.

Figure 3.3.  Maximum ESR vs Shunt Capacitance, C0 for 25 MHz XTAL

A high Q implies a better close in phase noise. It also means less frequency shift for a change in oscillator load capacitance and less shift due to other external factors such as oscillator supply voltage. Higher ESR reduces Q.

CL (Load Capacitance): This is the additional capacitance needed to load the XTAL for proper oscillation. This specification should match the loading provided internally by the built-in Si534x/7x/8x/9x oscillator, usually 8pF. Mismatch of the loading capacitance shifts the XTAL oscillation frequency.

Drive Level: The power dissipated in the XTAL must be limited or the XTAL may become less reliable. The maximum drive level a XTAL must tolerate is usually specified in its data sheet in units of micro-Watts (µW). Power dissipated in the XTAL may increase for high-ESR XTALs.

Aside from these electrical specifications, XTAL vendors also specify mechanical performance and manufacturing information. XTAL dimensions  could  also  be  important  as  this  affects  where  the  XTAL  will  be  placed.  Smaller  XTALs  can  be  placed  close  to  the Si534x/7x/8x/9x and thereby reduce the trace length.

XTAL Physical Size

XTALs come in many sizes, and include both thru-hole components with leads as well as surface mount components. The most com- mon surface mount packages are rectangular 4-pin packages with a welded or soldered metal lid. Two of the four pins are used to connect to each side of the XTAL. The remaining 2 pins are connected to the XTAL shield pins on the Si534x/7x/8x/9x devices, usually labeled as “X1” and “X2” . These packages are specified in terms of the X and Y dimensions of the package. For example, a common case size may be specified either as “3.2 mm x 2.5 mm”, or simplified to “3225” . Similarly, there are 2520, 2016, 1612, etc., sizes. For the larger package sizes, usually there is little effect on the electrical parameters of the XTAL. However, at smaller sizes, the ESR and Q may be affected due to the physically smaller XTAL required to fit in these packages.

Steps to Choose the Right XTAL for your Application

1. The nominal XTAL frequency must match the value set in the ClockBuilder™ Pro (CBPro) frequency plan on the Application/Refer- ence page of CBPro. The Si534x/7x/8x/9x cannot operate in a stable way if the XTAL frequency is different.

2. The total XTAL variation taking all factors into account must meet the value specified in the Si534x/7x/8x/9x device data sheet to ensure the best performance.

3. The XTAL maximum ESR must be below the C0/ESR curve. Higher ESR XTALs may not start reliably over all conditions.

4. The XTAL CL should match the value given in the Si534x/7x/8x/9x data sheet to ensure the correct oscillation frequency. However, XTALs with up to CL = 12 pF can be used by adding extra capacitance externally.

5. The XTAL drive level must be specified high enough to operate at the value specified in the Si534x/7x/8x/9x data sheet to ensure long-term reliable behavior.

4. Appendix B—How to Select the Right XTAL Oscillator for your Application

Introduction to XTAL Oscillators

XTAL Oscillator (XO): This is the most basic oscillator type which has a XTAL and a driver circuit in the package. The frequency stabil- ity is in the order oftens of ppm. These are very cost effective.

Temperature Compensated XTAL Oscillator (TCXO): As the name suggests, the oscillator is compensated for the change in itstem- perature. From the properties of XTALs, we know that the frequency changes with temperature and load capacitance. In the case of a TCXO, the temperature effect is balanced by purposeful capacitive loading, which enhances the frequency accuracy compared to an XO. Close to 1 ppm of accuracy can be obtained, however, it comes at an additional cost.

Oven Controlled XTAL Oscillator (OCXO): This has an oven built into the package and, instead of compensating for the temperature effects, it heats the oven to the zero-ppm temperature of the XTAL. In this case, the XTAL used needs to have its zero-ppm tempera- ture higher than the expected ambient as the oven cannot cool the XTAL. These have a very high stability, in the order of ppband slow aging as well. There is also a double oven version of this oscillator, namely the oven controlled OCXO which places the entire OCXO inside the oven to maintain the temperature. The oven and the control circuit add significant cost to the OCXO and are usually the most expensive amongst the oscillators.

Voltage Controlled XTAL Oscillator (VCXO): This is an extension to the XO with additional tunability. The frequency of the VCXO can be adjusted within 100s to 1000s of ppm by applying a control voltage, however, the tuning range is not as wide as a VCO. These oscillators are usually used as a reference to the 2nd PLL in a cascaded PLL. The cost for these oscillators falls somewhere between an XO and a TXCO.

The table below summarizes the difference between different types of oscillators.

Table 4.1. XO Comparison

Similar to the process for choosing a XTAL, the XO also needs to be evaluated for its properties and performance versus the require- ments.

Data Sheet Electrical Specifications

Frequency: The frequency of operation is determined by resonance of the XTAL inside the oscillator. Oscillators come in various fre- quencies ranging from kHz to MHz.

Frequency Accuracy and Stability: In timing and synchronization applications, frequency accuracy is one of the major concerns. Even small frequency deviations can cause a loss of sync. Hence, it is of utmost importance that the frequency remains stable over time and temperature.

This error is defined in terms of ppm (parts per million) or ppb (parts per billion).

ppm error = ((Actual frequency – ideal frequency) / ideal frequency) x 10⁶ 

ppm error = ((Actual frequency – ideal frequency) / ideal frequency) x 10⁹

The factors that contribute to this error are:

Initial Tolerance: This is due to the XTAL inside the oscillator. The imprecision of the cut and uneven width of the XTAL leads to an inherent frequency offset. This is defined at room temperature of 25 °C.

Temperature Stability: The variation arises due to the XTAL. The data sheet spec indicates the minimum and maximum variation above and below the 0 ppm temperature. For a simple XO, the stability follows the XTAL’s 3rd order temperature curve. The maximum deviation is in tens of ppm.

For the TCXO, this 3rd order curve is compensated by changing the loading capacitance. Thus, TCXO has a better temperaturestabili- ty over a simple XO, in the order of 0.1 ppm. The OCXO has the best temp stability as the XTAL inside the oven is maintained around its 0 ppm temperature. The accuracy of OCXO is around 0.01 ppm.

Supply Voltage Sensitivity: The change in the nominal frequency due to power supply variations defines this sensitivity. Usually, ±5% of supply voltage variation is tolerated and any noise in the power supply directly elevates the output phase noise. Thus, it is always recommended to use a clean and filtered power supply. The OCXO have a sensitivity in tens of ppband TCXO typically have it around 50 ppb. For an XO, it is usually combined with the overall accuracy spec indicating that it’s not very significant.

Load Sensitivity: The change in the load capacitance influences the nominal frequency, although not significantly. For a ±10% of the load condition change (standard load is usually 10 pF || 10 kΩ), the change in frequency (in ppb) defines load sensitivity. This value is tens of ppb for an OCXO and hundreds of ppb for a TCXO. For an XO, it is usually combined with the overall accuracy spec.

Reflow Sensitivity: The oscillator is subjected to high temperature followed by a cool down during reflow soldering. This can cause a frequency shift called thereflow sensitivity. It is expressed in ppm.

Aging: The XTAL inside the oscillator is an electromechanical

Output Characteristics: The output can be a differential or a single-ended type. All the Si53x/4x/7x/8x chips have a differential input for the Inx and XA/XB pins. A differential signal helps reduce the common mode noise. However, a low cost single-ended output XO can also be interfaced using an attenuator circuit to limit the maximum swing. Refer to section 5 of application note, ("AN905: External References: Optimizing Performance) for more details. A slew rate of 400 V/s (minimum) on the XA/XB pins is recommended to attain the best phase noise performance from the chip. When using the attenuator circuit to curtail the swing, care must betaken so that the load impedance by the circuit meets the oscillator load specifications.

Operating Temperature: This is the range of temperature which guarantees the operation of the oscillator per the datasheet specs. Operating temperature range should accommodate the system temperature range.

Power: The power consumption is added to differentiate between the OCXO and other oscillators. Since the OCXO has an oven built in, it initially consumes high power to heat up till the frequency settles. Since the oven is always present, the overall power consumed by OCXO is higher than others. Sometimes, OCXO and TCXO have a control voltage pin similar to VCXO that can be used to pull the frequency and thus needs an additionallow noise power supply.

Steps to Choose the Right XTAL Oscillator for your Application

1. Choose the type of oscillator you need for your application. You can use Table 4.1 XO Comparison on page 13 as initial guidance.

2. Table 4.2 on page 16 outlines the important oscillator specifications you should consider for different applications.

Table 4.2.  Oscillator Specifications

3. The peak-to-peak amplitude should be verified and an attenuator should be used if needed. See the reference manual for the Sili- con Labs device being used.

4. The slew rate needs to meet the data sheet specification for the Silicon Labs device being used.

5. The phase noise from the XO determines the output phase noise above the DSPLL bandwidth up to approximately 1 MHz. The XO needs to have approximately 20 dB margin in the phase noise to accommodate the additive phase noise from the device.

5.  Revision History

Revision 1.1

September, 2018

•  Added Si537x/9x devices coverage.

•  Added appendices explaining how to choose the right crystal and crystal oscillator for end application.

•  Removed discontinued parts from recommended part tables.

•  Added new parts to recommended part tables.

•  Added information in recommended part tables indicating part family to make these parts easier to find on vendor website.

Revision 1.0

January, 2017

•  Initial release.